1. Field of the Invention
The invention relates to a leak detector for a pressurized device. More particularly, the invention relates to a detector for detecting a gas leak from an intra-aortic balloon (xe2x80x9cIABxe2x80x9d) catheter system.
1. Description of the Prior Art
In counterpulsation therapy, a long cylindrical intra-aortic balloon is inserted percutaneously into a patient""s femoral artery. The balloon is then advanced until it is located in the descending aorta. Once in place, the balloon is inflated and deflated anti-phase to the pumping action of the patient""s left ventricle, i.e. the balloon is held inflated during at least a portion of diastole and is held deflated during at least a portion of systole. The IAB catheter is connected to a pump (IABP) for inflation and deflation of the balloon membrane on the distal end of the catheter.
IAB therapy is most effective when the transitions between the inflated and deflated states of the balloon membrane are rapid, typically, less than 0.1 second. When inflated and deflated in this manner, coronary blood flow is increased and cardiac work is decreased. This lifesaving therapy supports the heart during ischemic events or cardiogenic shock.
When sized for adult patients, the IAB balloon typically has a volume of 40 cubic centimeters. A small diameter catheter having an outside diameter of less than 0.1 inches (2.5 mm) is integral to the IAB. The catheter provides pneumatic access to the IAB. By design, the catheter""s diameter is small to reduce its impact upon blood flow in the patient""s femoral artery. As a result, the catheter""s small internal diameter impedes the flow of shuttle gas to and from the IAB balloon. Also, the catheter""s pressure drop at the catheter and balloon junction is large because the gas velocity in the catheter is high, approximately 500 ft/sec.
To reduce the impact of these effects: (1) helium gas is used as the working (shuttle) gas to inflate and deflate the IAB; and (2) a larger diameter xe2x80x9cextensionxe2x80x9d catheter is used to interconnect the IAB""s catheter to the intra-aortic balloon pump""s pneumatic port. The extension catheter""s diameter is larger, and thus, it""s pressure drop is lower.
During the pumping process, the shuttle gas is pressurized and de-pressurized each heartbeat by the IAB pump. The sources of the pressure changes are the pump and the restriction of the IAB""s catheter. As a thermodynamic consequence, on each inflation of the IAB, heat energy is stored in the shuttle gas, and on each deflation, heat is released.
Consequently, after each inflation or deflation event, the shuttle gas pressure changes as it attempts to thermally equilibrate with its environment. Typically, the pressure decays toward the appropriate average shuttle pressure in an exponential manner with a time constant on the order of approximately 1 second.
During IAB therapy, thermal equilibrium is not achieved because the durations of IAB inflation and deflation are too brief, i.e. the shuttle gas is re-compressed before it xe2x80x9crecoversxe2x80x9d from deflation and vice versa. The durations of the deflate and inflate intervals, are approximately 0.375 seconds at a typical patient heart rate of 80 beats per minute. The duration of the deflate interval is defined as the duration the balloon remains in the deflated state during systole plus the amount of time it takes for the balloon to deflate. The duration of the inflate interval is defined as the duration the balloon remains in the inflated state during diastole plus the amount of time it takes for the balloon to inflate.
If the shuttle gas leaks out of the IABP system, the IAB will not fully inflate. This diminishes therapy and can be harmful if gas is lost to the patient""s blood stream. Accordingly, there is a need for detection of gas leaks from the IAB system""s shuttle gas system. Detection systems have been incorporated into most existing IABP systems.
In principle, detection of gas loss appears straightforward. In accordance to the Ideal Gas Law, the quantity (mass) of a gas in a known volume can be determined by measurement of its static pressure, and temperature.
Accordingly, for leak detection using the Ideal Gas Law, IABP systems have a shuttle gas pressure sensor. To meet the Law""s xe2x80x9cknown volumexe2x80x9d requirement, the shuttle gas pressure is measured when the IAB is in its deflated state. This assures that the gas resides in the more stable and predicable geometries of the catheter(s) and intra-aortic balloon pump""s drive.
To meet the Law""s static pressure criteria, the shuttle gas pressure is measured as xe2x80x9clatexe2x80x9d as possible after IAB deflation. This maximizes the time interval available for IAB deflation. When the IAB is fully deflated, the shuttle gas is no longer moving and the xe2x80x9cstaticxe2x80x9d criteria is met. This corresponds to measuring the pressure just prior to IAB inflation.
The Law""s final criteria, measurement of shuttle gas temperature, is more difficult to adequately satisfy. This is because the shuttle gas temperature is the sum of two components, a local ambient temperature component and a thermal transient component, due to gas compression and decompression. Temperature sensors with the necessary speed of response to measure the thermal transients are fragile and expensive. For this reason, shuttle temperature is not measured by most IAB systems.
When the Ideal Gas Law is used for leak detection, and the effect of temperature is ignored, it is mathematically equivalent to assuming that the gas temperature is constant. In the case of local ambient temperature (average shuttle gas temperature), this is likely to be true if leak detection comparisons are limited to readings which were taken close in time. This is true if one presumes that the ambient""s effect upon the average temperature of the shuttle gas is slow, i.e. on the order of minutes.
However, in the case of the thermal transient component, it is not sufficient to compare heartbeats taken at similar times. An additional criteria must be added to avoid false alarms. This is a consequence of the thermal transient""s effect upon shuttle gas pressure. Specifically, after the gas is decompressed, its pressure exponentially decays toward the average shuttle gas pressure level. Typically, a leak detection pressure measurement is taken before this decay process is complete. As a result, the pressure reading has a transient component whose value depends upon the time when the reading was taken, relative the decompression event. Comparisons of pressure readings with different decay times result in false leak detection alarms, unless the alarm""s limits are made larger, and thus less sensitive, to exclude these errors.
Gas loss alarms can be absolute or relative. An absolute alarm compares the current gas pressure against a fixed pressure limit. To avoid false alarms due to the variability of temperature and volume, the absolute alarm limits must be large, on the order of three to five cubic centimeters per hour.
U.S. Pat. No. 3,698,381, issued to Federico et al., is an example of an absolute alarm system. Frederico et al. disclose an absolute leak detection method for an intra-aortic balloon catheter which involves monitoring the pressure of the shuttle gas just prior to inflation of the balloon. Leaks are detected on a beat-to-beat basis by comparing the measured pressure to fixed alarm limits. If the pressure of any single heartbeat is outside the fixed alarm or prescribed limits an alarm is declared. As discussed, the leak detection disclosed by Federico et al. is likely to cause false alarms because the effect of temperature is completely ignored.
A xe2x80x9crelativexe2x80x9d or differential gas alarm checks for gas loss from a known datum. The datum is an initial shuttle gas pressure measurement taken when the system is deemed leak free. After this initial datum is taken it is compared to subsequent pressure readings to determine if gas loss has occurred. An alarm is issued if the difference between the datum and a new reading exceeds a predefined limit.
The sensitivity of a relative gas alarm can be much higher than the absolute alarm because the initial datum implicitly includes the effects of current ambient temperature. This initial datum also implicitly captures the effect of tolerance of volumes. However, unless proper steps are taken, it does not include the effect of the thermodynamic temperature transient component induced by pumping.
The present invention comprises a differential leak detection method for an IAB catheter for detecting the loss of shuttle gas due to an IAB perforation. Perforations are typically due to abrasion of the IAB membrane by aortic plaque, and occur after an initial, leak free interval of pumping. The detection method accounts for the effects of the thermodynamic temperature transient induced by pumping by comparing shuttle gas pressure readings, taken just prior to IAB inflation, with similar thermodynamic histories, i.e. similar equilibrium times.
Accordingly, it is an object of the invention to produce a leak detector for an IAB system capable of making an accurate leak detection, by measuring changes in the gas pressure, despite thermodynamic variations in the shuttle gas temperature and the mechanical tolerances on pump volumes.
The invention is a leak detector for an intra-aortic balloon pump system comprising a processor, a pressure sensor, and optionally a temperature sensor. Gas leaks from the intra-aortic balloon pump system are detected by comparing shuttle gas pressure readings, taken just prior to IAB inflation, with similar thermodynamic histories, i.e. similar equilibrium times.
To the accomplishment of the above and related objects the invention may be embodied in the form illustrated in the accompanying drawings. Attention is called to the fact, however, that the drawings are illustrative only. Variations are contemplated as being part of the invention, limited only by the scope of the claims.